This invention relates to a multiple laser arrangement with phase-coupled laser diodes, and, it relates, more particularly, to a multiple laser arrangement wherein means for suppressing undesired transverse or ring modes is disposed at the lateral edges or extremities of the laser arrangement.
Many applications make it desirable to use phase-coupled laser diode arrays having a continuous wave power output in the watt region. This typically requires laser arrays with a plurality of single stripes.
A multiple laser arrangement enables the coupling of, for example, radiated power into a monomode or single mode optical fiber wherein the radiated power is on the order of approximately five to ten times greater than that in the case of a single laser. When coupling the radiated power into a monomode optical fiber, for example, a laterally coupled laser arrangement must have suitable radiation characteristics in the fundamental mode with a single lobe. Laser arrangements with a metallized standing waveguide structure, also referred to as MCRW lasers, permit lateral coupling of such lasers.
Phase-coupled laser diode arrays oscillate as the number of laterally coupled single stripes increases or as the ratio of the laser array width to laser resonator length increases above a value of 0.5 (when the laser reflector is asymmetrically coated with one-quarter wavelength layers and one-half wavelength layers for the purpose of unilaterally increasing the differential efficiency), which is preferred when operating low-loss internal oscillation modes. In such an arrangement, the decoupling of the laser radiation from the multiple laser arrangement (laser array) can, to a large extent, be prevented.
This effect of the widespread failure to prevent the decoupling of laser radiation has hitherto restricted the size and, at the same time, the maximum power output of conventional phase-coupled laser arrays.
The assembly and effect of a multiple laser arrangement (laser array) are described in Electronics, July 22, 1985, page 25. In this article, a multiple laser arrangement is described with forty single stripes wherein each individual stripe measures 400 .mu.m long and 3 .mu.m wide. Further, a gallium aluminum arsenide multiple layer is applied to a gallium arsenide substrate. The entire multiple laser arrangement is on a chip measuring 0.4 to 0.5 mm wide and 100 .mu.m thick. The typical center-to-center distance between stripes is 10 .mu.m. This multiple laser arrangement emits power into two lobes which are arranged to form an angle between them within seven degrees. Each lobe has a width of less than 2.degree.. Well-defined divergence of the radiated power results from phase-coupled operation of the multiple laser arrangement. The multiple laser arrangement can be produced either in a double hereto structure or in a MWQ (multiple quantum well) structure or even in another type of structure. The substrate layers can be produced by means of a metal-organic vapor-phase epitaxy (MOVPE) or by means of a liquid phase epitaxy or by means of another suitable technique.
With the conventional multiple laser arrangements, the partial suppression of internal oscillation mode is made possible by the fact that V-shaped etched grooves are produced as a lateral laser array limitation, or the fact that ring mode feedback is reduced by damage to the lateral chip surface unavoidably produced by sawing or other mechanical processes.
However, laser arrays with a one-quarter wavelength coating (antireflection layer applied to a laser reflector side so as to increase the differential efficiency) already show that, with a ratio of laser array width to laser resonator length of 0.45, for all practical purposes radiation decoupling no longer exists, even when the lateral edges of the laser array chips have been beveled by means of V-shaped etching.